Photoelectric exposure meter



Sept. 28, 1948. G MAISCH 2,450,288

\ PHOTOELECTRIC EXPOSURE METER Filed April 19, 1947 2 Sheets-Sheet 1 INVENTEOR BY M1 9% W I A TORNTZYS Sept. 28, 1948. G. MAISCH PHOTOELECTRIC EXPOSURE METER 2 Sheets-Sheet 2 Filed April 19, 1947 vs R NF E & n sw N m y W a my TORNEYS Patented Sept. 28, 1948 PHOTOELECTRIC EXPOSURE METER George Maisch, Sea Cliff, N. Y., assignor to Powers Photo Engraving Company, Glen Cove, N. Y., a corporation of New York Application April 19, 1947, Serial No. 742,543

19 Claims. 1

This invention relates to an electrical apparatus and particularly to electric timing means for controlling the duration of exposure in photographic or other processes where the integrated amount of light over an operating cycle is important. The invention is particularly adapted for photographic copying in process camera work where the'exposure limits are relatively narrow. However, the invention may be used to control the duration of exposure to light sources rich in ultraviolet, such as mercury arcs for use in sun-tan and medicinal applications. Other applications of the invention will be apparent to those skilled in the art.

In processes where chemical or photo-chemical light action is desired, the integrated value of light over a predetermined time interval or, conversely, a predetermined integrated value of light over a variable time (if the light source varies in intensity) are both of critical importance. As a rule, processes such as photographic require a predetermined product of average light and time of exposure to attain desired results. Due to such variables as distance from and the intensity of a light source, it is advantageous to,

provide a means which will integrate the effective light over an exposure cycle to determine definite limits for an exposure cycle.

The invention hereinafter described provides a system which is simple, accurate, reliable and which is rugged and may be used by unskilled operators. The invention also provides a system having substantial accuracy over its entire range of operation and having a generally linear operating characteristic permitting simple calibration of the system for a number of ranges.

The invention, in general, provides a condenser shunted by a light responsive resistance, such as a photoelectric cell disposed in a system including a combined rectifier and amplifier and two sources of potential, only one of which is operative on the condenser at any one time. In a normal stand-by condition of the system in preparation for an operating cycle, the condenser is connected to one source of potential for charging. This source may be either direct or alternating, since the amplifier tube also functions as a rectifier during the charging of the condenser. During an operating cycle, the con-denser is isolated from the charging potential source and is connected in series with the input or control circuit of the amplifier across the terminals of a constant potential source which functions to clamp or determine the potential for th condenser terminal remote from the vacuum tube.

By suitable choice of the clamping potential with respect to the charging potential, the system will maintain the amplifier cut-01f during the operating cycle during which time the condenser will discharge. The discharge of the condenser to a predetermined point will terminate the operating cycle. The discharge may be confined to the linear portion of the curve so that calibration of the system is simplified.

The two potential sources are regulated to control accurately both the potential to which the condenser is to be charged and the reference potential to which one condenser plate is fixed. This not only provides accurate and sensitive operating characteristics for the system but also endows the system with an immunity to disturbing potentials and makes the system rugged. This is particularly important in the case of energization of the system from a conventional power source, such as a 110 volt, 60 cycle line.

For a more complete description of the invention, reference will now be made to the drawings where Figure 1 shows a circuit diagram of a system embodying the present invention. Figure 2 shows a circuit diagram of a modified form.

Transformer ID has primary H whose terminals l2 and [3 are connected to lines l4 and NS for supplying alternating current from any suitable power source, such as the customary 110 volt, 60 cycle supply. Line i5 is provided with switch NS for controlling the primary circuit. Transformer Ill has filament winding 18 connected to wires I9 and 20 for supplying the heaters of the various vacuum tubes used in the system, Transformer I!) also has filament winding 22 connected to leads 23 and 24 for energizing cathode 25 of rectifier 28. Rectifier 26 has anodes 21 operating in parallel and connected to line 29 going to tap 30 of high potential secondary winding 3|. Secondary win-ding 3| has terminals 32 and 33. Terminal 32 is connected through ourrent-limiting resistor 34 to grounded lead 35. Connected between line 48 and ground is any suitable means for providing voltage regulation. Thus, gas tube 3'! having anode 38 connected to line and cathode 39 connected to ground may be provided. Such regulator tubes maintain the peak potential at a constant value and are widely used in various systems.

Filament winding 22 for the rectifier has lead to connected thereto. Lead 40 is mid-way on the winding so that its potential is half-way between the potentials of the terminals of winding 22 and, thus, is constant relative thereto. Lead goes to terminal 4|, between Which terminal and grounded lead 35 there is connected condenser t2, Condenser A2 preferably has substantial capacitance and, as will be shown later, functions as a source of constant potential during an operating cycle. By connecting regulator tube 3! in series with rectifier 26, the desired voltage regulation between junction H and ground is obtained while providing the gas tube with protection against excessive current flow therethrough.

Connected to junction AI is a variable resistor consisting of resistor 53 and wiper it cooperating therewith. Wiper id is connected to one terminal of resistor i5 forming part of a second potentiometer. Resistor 65 has its other terminal connected to grounded wire The resistance network across condenser 32 preferably has substantial value so that, in combination with condenser 52, a resistance-condenser circuit is formed having a time constant for discharge purposes long in comparison to the period of a cycle of power supplied to transformer I 9. Cooperating with resistor 55 is wiper ll. Wiper G? is connected preferably through resistor 38 to junction as and thence, by lead 59, to terminals 5!, 52, 5-3, and 54 Terminal 5I has connected thereto one terminal of condenser 56. The other terminal of condense E8 is connected to line 57. Condenser 56 preferably has a small or moderate capacitance and is adapted to control the duration of an operating cycle for the system.

In order to increase the flexibility of the system, condensers 59 and 69 may be provided. Thus, condenser 59 is connected between junction 53 and terminal SI of selector switch 6 Ia. Condenser 69 is connected between junction 54 and terminal 62 of the selector switch. Discharge resistor 53 is connected between junction 52 and terminal I35 of the selector switch. The selector switch has fan-shaped contact 66 cooperating with contacts (SI and 62 and small contact 63' cooperating with contact 65 and with contacts 63 and 69 on opposite sides of contact 65. Contacts 68 and 69 are connected to lead 57. By proper design of the selector switch, it is possible to have condensers E8 and 66) both shore-circuited through resistor 63, as shown, or to have condenser 59 connected in parallel to condenser 56, or condensers 59 and 6G both connected in parallel to condenser 56. Cther means for varying the effective capacitance availabl between lead 56 and lead 5'! may be provided.

Connected between leads 5% and 51 is a lightresponsive means such as photoelectric cell Ill. Cell 'IIJ may be one of several types and has anode 'II connected to lead 59 and cathode 12 connected to lead Ill. Cell Iii is housed in light-tight box I3 having adjusted diaphragm i l and adapted to be exposed to the light used in the photographic or other process. The cell may be exposed either to the light source, may receive light reflected from the photographic film or sensitive paper, or may be exposed to light transmitted through a photographic negative, for example. The photoelectric cell may conveniently be disposed is fixed relation to the film or sensitive paper, upon which the desired light is focussed, so that the cell itself is automatically afiected with the photographic material to variations in distance from the light source or variations in intensity or quality of the light source.

Lead 57 is connected to control grid I5 of vacuum tube amplifier l6. Amplifier I6 is preferably a simple triode, although tubes having more electrodes may be used. Amplifier "I5 has filamentary heater TI connected to leads I9 and 2G for energization. Amplifier I5 has cathode I8 energized by heater Ii, cathode I8 being connected to wire I 9 which is grounded as far as the system is concerned. Amplifier it has anode 89 connected through load resistor 8| to wire 82 going to tap 39 of the high potential transformer winding. Connected across load resistor 8I is condenser 83.

Anode so is also connected to wire 83 going to control grid 85 of amplifier 86. Amplifier 86 is preferably of the type designed to have substantial space current and, while shown as a beam power tube, may be any other suitable type. Amplifier 86 has heater 8? connected to lines I9 and 2S. Amplifier 86 has cathode 88 and beam control electrodes 39 and 93 connected together in the usual fashion. Cathode 83 is connected to line SI going through junction 32 and switch 93 to junction 94. Line 29 is connected to junction 94. Amplifier 85 has accelerating grid 55 connected through dropping resistor 97 to wire 98. Wire 98 is connected to terminal 33 of the high potential winding. Between wire 98 and anode I69 of amplifier 86 are connected condenser Id: and relay winding I82, the latter forming part of relay IE3. Relay I03 has movable contacts Hm and I respectively. Movable contact led is normally open and, for convenience, is shown as bearing against dead back contact I96 in its normal position. Cooperating with movable-contact IE4 is off-normal contact IB'I connected by wire I88 to junction IE9 of load Hi). Load H5 may be a shutter control winding whose other terminal III in connected by wir H2 to terminal I2 of primary winding II. Terminal I89 is connected by wire I I3 and switch I I I to line I5 of the power supply, this latter connection providing a manual shutter control independent of the automatic control exercised by the system. Pilot light IE5 across load H6 is provided to show the condition of the load.

Referring back to relay i533, movable contact 95 normally bears against back contact III; connected by wire I It and switch I28 to junction 49. Movable contact 5 in the ofi-normal position cooperates with forward contact I22 connected by lead I23 to junction 92. Between wire 9i and accelerating grid 96 of amplifier 85 is switch I25 for manually terminating an exposure cycle.

Switches 93 and I28 are preferably mechanically interconnected so that they may be operated together. Switch 93 is normally open, and switch I2EI is normally closed. l'hey are biased to the position shown in full lines and are adapted to be moved to the dotted line position momentarily by an operator to initiate an exposure cycle.

Wiper 41 may be provided with indicator I2! cooperating with scale I28. Scale I28 may be suitably calibrated to indicate exposure time. It is understood that the calibration of scale I28 will depend upon the adjustment of potentiometer wiper 44. Thus, the potentiometer wiper may be provided with one or more predetermined calibrated positions. A suitable multiplication factor for the potentiometer wiper positions may be pro vided for scale I28.

The operation of the system is as follows. Assume that switch I6 is closed thus energizing primary II of transformer I8. Filament winding I8 will be energized, this being indicated by pilot light I26. Thus, amplifiers "I6 and 86 will have their cathodes energized. At the same time, winding 22 will have current therein to energize rectifier cathode 25. It will be apparent that, when terminal 30 is positive with respect to terminal 32 of the high potential winding, current other condensers, may be ating range of the system. In actual practice,

resistor connected between grid I 'tential present at will flow through rectifier 28 and also through regulator tube 31.

Rectifled current passing through 26 will go alonglead 40 to junction 4|. From junction 4I. charging current will flow to condenser 42 and thence to ground. After a short time, condenser 42 will become charged to the maximum potential available. Switches 93, I and I are in the full-line position. For simplicity, the selector switch will be assumed to be in the position shown where only condenser 56 will be used. Since relay I03 is in the normal position shown in the drawing and since switch 93 is open, power tube 86 cannot conduct. During this time, positive surges along line 29 will go through junction 94, relay contact I05, normal back contact II8, lead H9 and normally closed switch I20 to junction 49. From junction 49, positive surges will go to terminal 5|. Upon the occurrence of each positive surge, charging current can flow into condenser 56. The circuit goes on from condenser 56 to line 51 and control grid 15 of vacuum tube I6. The flow of charging current for condenser 56 will maintain grid 15 at or above cathode potential and grid current to cathode 18 will be established. Thus, for surges when terminal 30 is positive to terminal 32 of the high potential winding, 9. complete charging circuit for condenser 56 will be established. Resistor 34 will limit the charging current to a safe value. It is preferred to design the time constant of the charging circuit for condenser 56 so that condenser 56 will be charged in several cycles. For a cycle supply, the time constant may be equal to the period of about two or three cycles. This, however, is exemplary and, with the addition of varied to suit the opersome current from grid I5 to cathode I8 will generally what may be considered, for all practical purposes, iully charged. This current will be due to the fact that the photo cell may conduct to some extent and the fact that the grid potential tends to remain at or slightly above the potential of the cathode. Leakage through the condenser, as well as the leakage resistance present between the grid and cathode of tube I6, will also be a factor. If desired, the leakage across the terminals of condenser 56 may be stabilized by a suitable grid and cathode Since condenser 56 is charged, the system may be considered to be in a stand-by condition and ready for an operating cycle. Assuming that an exposure is to be made, switch 93 is momentarily closed and switch I20 is simultaneously opened. As will be explained later in connection with the charging circuit, the opening of switch I20 serves to definitely cut-off tube I6 and thus to cut off the flow of load current through resistor 8I. In practice, condenser 83 is'small enough that it discharges quicker than switch 93 and switch I20 can be opened. Thus, the potential of control grid 85 and cathode 88 are equalized to the poent that, with the closure of switch 93, a complete circuit for space current through the power tube is established. During each cycle, space current tends to fiow through power tube 86. When power tube 86 conducts, the current through relay winding I92 will suffice to close relay contacts I04 and I05. The closure of relay contact I04 against back contact I01 will close the load circuit which, in this instance, conflow even after condenser 56 has become terminal 30. It will be apparthe positive portions of 7 sists of winding IIO. Condenser IIII will filter out the current surges. Thus, if winding I I0 controls a shutter, it will be apparent that an exposure cycle will thereupon be initiated.

Relay contact I05 is pulled to forward contact I22 and thus effectively short-circuits switch 93. Hence, when switch 93 is opened, the system will continue to operate during an exposure cycle. Thus, the switching position in itsoil-normal position permits the system to go through an operating cycle. When movable contact I05 is away from normal contact from terminal 30 of the transformer winding to condenser 56 will be broken. It is apparent that the potential of junction 49, when the relay is in a normal position, will vary from a high positive to a high negative value with respect to ground. Hence, when the relay breaks the charging circuit for condenser 56, the instantaneous potential of junction 49 may, at the moment of break, be anywhere between two extreme values. However, resistor 48 is high enough so that the tendency to charge or discharge condenser 42 will be negligible during the time that junction 49 is going through its potential extremes. Thus, during normal or charging circuit conditions, the constant potential source is practically isolated from condenser 56. Upon the break of the char ing circuit to condenser 56, the potential of junction 49 quickly reaches a value determined by the position of wiper 41. The potential of wiper 41 to ground is maintained constant. Thus, junction 49 is clamped to a fixed potential above ground.

It is apparent that the potential of junction 4i only differs from the peak potential at terminal 30 by the drop through rectifier 26 and load resistor 34. Thus, junction 5I will assume a potential positive with respect to ground but at some intermediate value between ground and the positive peaks at terminal 30 depending upon the position of wiper 41 in the network. The potential of grid I5 will drop below ground by a corresponding amount, depending upon the position of wiper 41. The amount of this drop will be illustrated later in connection with a circuit having specific values. The drop in grid potential will sufiice to cut tube I6 ofi.

During the course of an exposure or operating cycle, photo-electric cell I0 functions as a resistor Whose value is a function of the light intensity at the cathode. It is understood that, in case cell I0 is of the type generating a potential, the action is similar. In such a case, the potential generated by the cell may be considered as a negative resistance having a value functionally related to the light. Cell I0 thus provides a discharge path for condenser 56. Since the potential of junction 5I is maintained constant with respect to ground, it is apparent that the potential of grid. I5 will rise. During the exposure cycle, tube I6 remains cut off. However, when the discharge of condenser 56 has run a predetermined course, the potential of control grid 15 will rise to a cut-in value for amplifier I6. Upon cut-in of amplifier I6, the potential of anode will drop. This will cause the potential in line 84 to drop and thus result in a drop in potential of control grid 85 of power tube 86. The drop in potential across load resistor BI is so designed that tube 86 will thereupon be cut off.

When tube 86 is cut-off, current through relay winding I02 stops, and the relay assumes its normal position, This breaks the load circuit to winding H0 and determines the endof an ex posure period or other operating cycle.

II8, the charging circuitdenser.

The difference in potential between wi er 41 and ground or cathode 118 is a measure of the potentlal through which control grid is depressed below cut-oil. Assuming for the sake of simplicity that tube 16 cuts-off when the control grid becomes negative to-the cathode (this is merely an assumption and is not essential to the opera tion of the system), the higher the potential of wiper 41 above ground-theless will be the drop of potential of control grid 15 below ground. Thus, as a practical example, assume that the potential across condenser 42 is 140 volts. Also assume that the potential across condens r 5 is 140 volts. In the system disclosed, the potential across condenser 56 may be somewhat higher due to the drop through rectifier 20. However, this is not important, and the potential across the two condensers may b equal, or the potential across condenser 56 may be greater or less than that across condenser 42,.

Assuming, however, as above, that junction 4| is plus 140 volts, junction 5| will be a bit less than 140 volts when condenser 56 is fully charged. Now assume that the potential of wiper 4! is plus volts. cycle and during the courseof this operating cycle, the potential of junction 5| will be clamped to plus 30 volts. However, at the beginning of the operating cycle, grid 75 will drop to minus 110 in order to maintain the 140 volts across condenser 56.

It follows that condenser 56 will have to dis charge to the point where the potential across the condenser will be 30 volts. Thus, a long exposure cycle will occur.

If the potential of wiper 47 is adjusted to plus 100 Volts, it follows that grid 15 will be dropped volts below ground.

As disclosed herein, resistor 43 and wiper 44 -forming an auxiliary potentiometer are disposed on the high potential side of resistor 45. With such an arrangement, it is evident that the operating range for condenser 55 is in the upper portion of the potential discharge curve for this con- It is possible, however, to dispose resister 43 and wiper 44 onthe low potential side of resistor 45 so that, in the operating range of the system as controlled by wiper 41, condenser 55 will never discharge completely. While such a modified system functions in an efiicient and satisfactory manner, it has been found that such a modified system may be somewhat sensitive to variations in tubes incident to replaceemnt. It is possible that capacitance to ground of various components of the system, as well as conducting characteristics of photo-cell 10, may account for such increased sensitivity.

The operating characteristics and time constants of the various circuits may most easily be discussed in connection with a system having spe cific constants and potentials. Thus, in one system, regulator tube 31 was type VR150 available in the market and sold by Radio Corporation of America and others. Such a tube tends to limit the potential across th electrodes to a peak of 150 volts. Rectifier 26 was a 5V4 of the full-wave type, the two portions of the rectifier being connected as shown to provide a simple half-wave rectifier. Amplifier 16 was a 6J5, while power tube 86 was type. 6V6. Photoelectric cell 10 was type 929. Resistor 34 had a value of about 5000 ohms, Resistors 43 and 45 each had values of 5000 and 10,000 ohms respectively.

Resistor 48 had a value of 100,000 ohms, re-

Upon the initiation of an operatin -8 sistor 91 had a value .of about 30,000 ohms, and resistor 8| had a value of about one megoh'm.

Condensers c2 and NH each had values of 12 microfarads. Relay I03 was a simple relay which could be operated withspace current through the beam tube. Condenser 56 had a capacitance of about one-half microfarad, while condenser 83 had a capacitance of about .05 microfarad. The R. M. S. open circuit potential of terminal 30 with reference to terminal 32 was .140 volts, and the corresponding potential of terminal 33 with respect to terminal 32 was 440 volts. .Condenser 59 was .5 microfaradand condenser 60 wasel microiarads.

In the practical system, identified above, the time constant for charging condenser 56 alone is 2500 microseconds. With a sixty-cycle power source, a period for the power source is almost 17,000 microseconds. 7 Thus, condenser 56 can be completely charged in about ten cycles. The time constant for charging condenser 42 is about 60,000 microseconds while the discharge circuit'time constant is between about 120,000 and about 240,000 microseconds. The time .constant for the discharge circuit of condenser 56 will depend upon the light falling on cell 10, assuming switch 93 is open and tube 76 cut off. However, as long as tube 16 is cut off, the time constant of the shunt circuit comprising wiper 47, resistor 48, the condenser and photoelectric system, grid to ground is theoretically infinite and practically very long. Even when grid 15 is above cut-off, the time constant oi. this circuit is very large. Hence, it is evident that wiper-4'! carries little if any current, and the potential of Wiper 4! during an exposure cycle is constant.

It is possible to have the resistance network around condenser 4:! much higher, say about 100,000 ohms or the like, and eliminate resistor 48. However, the arrangement shown is convenient for potentiometer control and satisfactory tolerances may be easily maintained. Furthermore, there is sufiicient current flow around condenser 42 so that any sudden shifts in potential at wiper 4? are quickly dissipated.

For expanding the range of operation of the system, condensers 59 and 50 may be relied upon or diaphragm 5'4 may be adiusted to control the amount of light impinging on the oathode of the photoelectric cell. Thus, condenser 59 may be about one-half micro-farad and condenser may be four micro-iarads. Other values, however, may be selected. Resistance 63 may have any desired value, as 5000 ohms, to discharge the condensers when they are not to be used.

It is evident that, if diaphragm 14 is closed to reduce the amount of light on cell 10, the rat o of diaphragm areas may be used as a multiplying factor for increas ng the operating range of the system. It is also possible to use the diaphragm in a manner to bring the cell response to a desired level to calibrate the entire system. This is particularly advantageous with the present system, since all variables are linear and provide a variety of operating ranges.

At any t me during an operatingcycle, the cycle may be terminated at will by closing switch I25. This drops the potential of accelerating grid 96 down to cathode potential and cuts on the beam tube. The relay thereupon opens and the system reverts to normal.

Referring now to Figure 2, a modification is shown wherein the high side of condenser-56 is point 94. By virtue of this change, greater precision in setting the system for zero or short exposures is possible. This change, if desired, may be incorporated in the system shown in Figure 1. An additional movable contact I05 is provided. Contact I05 normally bears against back contact H8 connected to movable contact I05. Movable contact I05 is connected by lead 50 to the high side of condenser 56. Wiper 41' is connected by wire 58 to forward contact I22. The back contact for movable contact I05 is unnecessary. Switch IZil' movable with switch 93 servesto short load resistor 8| and make the potentials of grid 35 and cathode 88 equal.

, A charging circuit for condenser 56 is established from movable contact I05 to contact IIB, movable contact I05 and wire 50. When manual switch 93 is closed power tube 05 becomes conducting and the relay locks in position. Thereupon, the charging circuit to condenser 56 is broken, and wiper 4'! is connected to condenser 56 through movable contact I05 and fixed contact I22. In other respects the modification shown in Figure 2 is generally similar to the embodiment illustrated in Figure 1.

In both circuits, it is necessary that the potential of wiper 41 or 41 with respect to the cathode of tube It be higher than the potential of-grid I5 to cathode 18 at cut-off. Thus, when condenser 55 discharges, grid I5 can cutin tube and terminate the operating cycle. In other words, the difference of potential across condenser 56 when fully charged must be greater than the potential between the tube cathode and cell anode at the beginning of an operating cycle due to the'constant potential source.

What is claimed is: v 1. In a system of the character described, a photoelectric cell having cathode and anode and exposed to light during an operating cycle, a condenser connected across said cell, a vacuum tube having at least cathode, control grid and anode, an output circuit including a load connected to the tube cathode and anode, a connection between the grid and cell cathode, two sources of potential at least one of which is a source of constant-potential, connections from the tube cathode to the negative terminal of said one constant potential source and to one terminal of the second potential source respectively, means including switching means having normal and off-normal positions for impressing selectively the potential oi the remaining terminal of each potential source upon the cell anode, said switching means in the normal position providing a circuit for charging said condenser with grid current furnished by said second potential source, said switching means in the oil-normal position during an operating cycle providing a circuit for clamping the cell anode potential to the positive terminal of said constant potential source while the cell discharges the condenser during the operating cycle, said clamping potential being above grid cut-off whereby said tube is cut-off i 10 theoperating cycle and cuts in to terminate the operating cycle.

2. The system of claim 1 wherein said load includes means responsive to space current through said vacuum tube for maintaining said switching means in oiT-normal position during an operating cycle after the initiation thereof and for returning said switching means to a normal position when said tube cuts in at the termination of an operating cycle.

3. The system according to claim 1 wherein means are provided for varying the potential of the positive terminal of the source of constant potential and wherein indicating means are associated with said varying means for indicating the duration of an operating cycle.

4. In a system of the character described, a photoelectric cell having cathode and anode and exposed to light during an operating cycle, a condenser connected across said cell, a vacuum tube having at least cathode, control grid and anode, an output circuit including a load connected to the tube cathode and anode, a connection between tne grid and cell cathode, a source or constant potential including a second condenser and resistance combination, a second source of potential, connections irom the tube cathode to the negative terminal of said constant potential source and to one terminal of the second potential source respectively, a potentiometer connection to the resistance of said constant potential source, means including switching means having normal and off-normal positions for impressing selectively the potential of the potentiometer connection and remaining terminal of the second potential source upon the cell anode, said switching means in the normal position providing a circuit for charging said first condenser with grid current furnished by said second potential source, said switching means in the off-normal position during an operating cycle providing a circuit connecting the potentiometer connection and cell anode for clamping the cell anode potential to said potentiometer potential while the cell discharges the condenser, said clamping potential being above grid cut-off during the operating cycle and cuts in to terminate the operating cycle.

5. The system of claim 4 wherein said potentiometer connection is connected to said cell anode through a high resistance in both switch positions, said potentiometer connection being substantially isolated from said cell anode in the normal position of said switching means.

6. The system of claim 4 wherein indicating means are provided for said potentiometer connection, said indicating means being adapted to show the duration of an operating cycle corresponding 'to the setting of said potentiometer connection.

7. In a system of the character described, a photoelectric cell having a cathode and anode and exposed to light during an operating cycle, a first condenser connected across said cell, a vacuum tube having at least cathode, control grid and anode, an output circuit including a load connected to the tube cathode and anode, a connection between the grid and cell cathode, a source of constant potential including a second condenser and resistance connected across said second condenser, a source of alternating potential, connections from the tube cathode to the negative terminal of said second condenser and to one terminal of the alternating potential source respectively, means including switching means during 7 having normal and off-normal positions for impressing selectively the potential of the remaining terminal of said alternating potential source and a point on said resistance upon the cell anode, said switching means in the normal position providing a circuit for charging said first condenser from the alternating current source with grid current, said switching means in the oiT-normal position during an operating cycle providing a circuit for clamping the cell anode potential to said point on said resistance While the cell discharges the first condenser, said clamping potential being above grid cut-oif whereby said tube is out 01f during the operating cycle and cuts in to terminate the operating cycle.

8. I'he system of claim I wherein potentiometer means are provided for selecting a point on said resistor and wherein indicating means coupled to said potentiometer are provided for showing the duration of an operating cycle.

9. The system of claim 7 wherein the circuit between said cell anode and point on said resistance includes a potentiometer connection and. high resistance, said circuit being closed in both positions of said switching means.

10. The system of claim 7 wherein means responsive to fiow of current is provided in said load for maintaining said switching means in off-normal position after the initiation of an operating cycle and for the duration of said operating cycle and for returning said switching means to the normal position when said tube cuts in to terminate the operating cycle.

11. In a system of the character described, a photoelectric cell having cathode and anode and exposed to light during an operating cycle, a condenser connected across said cell, a vacuum tube hawing at least cathode, control grid and anode, an output circuit including a load connected to the tube cathode and anode, a connection between the grid and cell cathode, a source of alternating potential, a rectifier and second condenser in series across said potential source, the common connection between the second condenser and rectifier having a positive polarity with the remaining second condenser terminal having a negative polarity, a connection between the tube cathode and negative terminal of said second con denser, a resistance network shunted across said second condenser, a potentiometer connection on said resistance network for determining a point on said network having a predetermined constant positive potential with respect to said vacuum tube cathode, said second condenser and resistance network having a time constant for discharging substantially greater than the period of a cycle from the alternating potential source, means including switching means having normal and ofi-nornnal positions for impressing selectively the potential at the rectifier anode and potentiometer connection upon the cell anode, said switching means in the normal position providing a circuit for charging the first condenser from the alternating potential across said source with said tube cathode and grid rectifying the current, said switching means in the off-normal position during an operating cycle providing a circuit for clamping the cell anode potential to said potentiometer connection while the cell discharges the first condenser, said clamping potential being above grid cut-on whereby said tube is cut-ofi during the entire operating cycle and cuts in to terminate the operating cycle.

12. The system of claim 11 wherein means are provided for maintaining constant the peak potential across said alternating potential source 7 showing the duration of an during portions of the cycle when the rectifier anode is positive.

13. In a system of the character described, a photoelectric cell having cathode and anode and exposed to light during an operating cycle, a first condenser connected across said cell, a vacuum tube having at least cathode, control grid and anode, an output circuit including a load connected to the tube cathode and anode, a connec tion between the grid and cell cathode, a source of alternating potential having one terminal connected to said tube cathode, a rectifier and second condenser connected in series across said alternating potential source, said rectifier having its anode connected to the other terminal of said alternating potential source, said second condenser having its negative terminal connected to the one terminal of said alternating potential source, a resistance network shunted across said second condenser, a potentiometer connection on said resistance network for determining a point having a predetermined positive potential with respect to the tube cathode, means for maintaining constant the peak potential across the terminals of said alternating potential source during those portions of a cycle when said rectifier can conduct, means including switching means having normal and off-normal positions for impressing selectively the potential of the rectifier anode and potentiometer connection upon the cell anode, said switching means in the nonmal position providing a circuit for charging said first condenser with current furnished by said alternating potential source and rectified by the cathode and grid of said tube, said switching means in the onnormal position during an operating cycle providing a circuit for clamping the cell anode potential to said potentiometer connection while the cell discharges the first condenser, said clamping potential being above grid cut-off whereby said tube is cut-off during the entire operating cycle and cuts in to terminate the operating cycle.

14. The system of claim 13 wherein a high resistance connection between said potentiometer connection and cell anode is provided in all switch positions.

15. The system of claim 13 wherein indicating means are associated with said potentiometer for operating cycle.

16. The system of claim 13 wherein means are provided for controlling the amount of light admitted to the photoelectric cell for determining the operating range of the system.

17. In a system of the character described, a photoelectric cell having cathode and anode, a first condenser connected across said cell, a first vacuum tube having cathode, control grid and anode, a connection between the grid and cell cathode, a source of alternating potential having ground, intermediate and high potential terminals respectively, a rectifier and second condenser connected in series across said intermediate and ground terminals, the grounded condenser terminal having a negative polarity, a regulator tube for controlling the peak potential connected across said second condenser for maintaining constant the positive peaks with respect to ground, a resistance network shunted across said second condenser, a variable potentiometer wiper cooperating with said resistance network for selecting a point on said resistance network positive with respect to ground, said wiper having indicating means to show the duration of an operating cycle, said second condenser and resistance network having a long time constant, a

connection including a high resistance between said potentiometer wiper and cell anode, a relay circuit connecting the intermediate terminal and cell anode, said circuit when closed being adapted to provide charging current for said first condenser, said cell anode being adapted to be clamped to the potentiometer wiper when said relay circuit is open, a second vacuum tube having cathode, control grid and anode, a circuit in cluding a manual switch for connecting said second tube cathode to the intermediate terminal and second tube anode to the high potential terminal, a connection between the control grid of said second tube and the anode of said first tube, a load resistor connected between the anode of said first tube and said intermediate terminal, relay means in the cathode-anode circuit of said second tube, said relay means having normal and off-normal positions corresponding to non-conducting and conducting conditions respectively in said second tube, said relay having circuit connections for maintaining open the relay circuit between the intermediate terminal and cell anode when said relay is in off-normal position and for maintaining said relay circuit closed when said relay is in the normal position, said manual switch, upon closure being adapted to cause the cathode-anode circuit of said second tube to move said relay toan off-normal position and maintain the charging circuit to said first condenser open with said first tube remaining at cut-ofi during the operating cycle and cutting in at the termination of said operating cycle to cut-off the second tube and return said relay to a normal condition.

18. The system of claim 17 wherein means are provided for determining the amount of capacitance across said cell to determine the operating range of the system.

19. The system of claim 17 wherein said cell has means for adjusting the amount of light thereon to determine the operating range of the system.

GEORGE MAISCH.

No references cited. 

